Field of the Invention
Exemplary embodiments of the present invention relate to light-emitting diodes and to light-emitting diodes having electrode pads, and more particularly, to high voltage and/or high efficiency light-emitting diodes.
Discussion of the Background
Gallium nitride (GaN) based light emitting diodes (LEDs) have been used in a wide range of applications including full color LED displays, LED traffic signals, and white LEDs.
The GaN-based light emitting diode may be generally formed by growing epitaxial layers on a substrate, for example, a sapphire substrate, and includes an N-type semiconductor layer, a P-type semiconductor layer, and an active layer disposed between the N-type semiconductor layer and the P-type semiconductor layer. Further, an N electrode pad is formed on the N-type semiconductor layer and a P electrode pad is formed on the P-type semiconductor layer. The light emitting diode is electrically connected to and operated by an external power source through these electrode pads. Here, electric current is directed from the P-electrode pad to the N-electrode pad through the semiconductor layers.
Generally, since the P-type semiconductor layer may have a high resistivity, electric current may not be evenly distributed within the P-type semiconductor layer, but may be concentrated on a portion of the P-type semiconductor layer where the P-electrode pad is formed. Electric current may be concentrated on and flow through edges of the semiconductor layers. This may be referred to as current crowding, and may lead to a reduction in light emitting area, thereby deteriorating luminous efficacy of a source. A transparent electrode layer having a low resistivity may be formed on the P-type semiconductor layer to enhance current spreading. In this structure, electric current supplied from the P-electrode pad may be dispersed by the transparent electrode layer before entering the P-type semiconductor layer, thereby increasing a light emitting area of the LED.
However, since the transparent electrode layer may tend to absorb light, the thickness of the transparent electrode layer is limited, thereby providing limited current spreading. In particular, for a large LED having an area of about 1 mm2 or more, there is a limitation on current spreading through the transparent electrode layer.
To facilitate current spreading within a LED, extensions extending from the electrode pads may be used. For example, U.S. Pat. No. 6,650,018, issued to Zhao, et al. discloses an LED that includes a plurality of extensions extending in opposite directions from electrode pads to enhance current spreading. Although the use of extensions may enhance current spreading over a wide region of the LED, current crowding may still occur at portions of the LEDs where the electrode pads are formed.
Moreover, as the size of the LED increases, the likelihood of a defect being present in the light emitting diode may increase. Defects such as threading dislocations, pin-holes, etc. provide a path through which electric current may flow rapidly, thereby disturbing uniform current spreading in the LED.
A light emitting device having a plurality of light emitting cells serially connected to each other in a single chip to operate under high voltage is disclosed in U.S. Pat. No. 7,417,259, issued to Sakai, et al.
Recently, light emitting devices capable of being powered by 110 V or 220 V domestic alternating current (AC) power sources have been commercialized. In such a light emitting device, chips each having a plurality of light emitting cells serially connected to each other are mounted in a package to be serially connected to each other, and each package may be used together with a bridge rectifier. Two to four light emitting diode chips, each of which includes sixteen to twenty light emitting cells serially connected to each other on a single substrate, may be connected in series within a package to be powered by 110 V or 220 V power sources.
Particularly, assuming that each light emitting cell is powered by a forward voltage of about 3.5V, three to fifteen light emitting cells are serially connected to each other in order to provide high voltage products, such as 12V, 24V, 36V, 48V, and the like, to which a switching mode power supply (SMPS) is applied. Since a power of about 3 to 11 W may be applied to high voltage products, relatively high electric current may be supplied to each of the light emitting cells. In this case, if the light emitting cells have excessively small sizes, current density may be excessively increased, thereby deteriorating reliability. On the contrary, if the light emitting cells have excessively large sizes, electric current may not be uniformly distributed, thereby causing current crowding and deteriorating light extraction efficiency.
FIG. 7 is a schematic plan view explaining a conventional high voltage light emitting diode and FIG. 8 is a schematic circuit diagram of FIG. 7. Here, three light emitting cells are connected in series.
Referring to FIG. 7 and FIG. 8, a conventional light emitting diode includes a substrate 21, light emitting cells C1, C2, C3, a cathode 37, an anode 39, interconnecting sections 41, first electrode pads 35, and second electrode pads 33.
The light emitting cells C1, C2, C3 are separated from each other on the substrate 21 and each includes a first conductive type semiconductor layer 23, an active layer (not shown), and a second conductive type semiconductor layer 27. Here, the first conductive type semiconductor layer 23 is an n-type semiconductor layer, and the second conductive type semiconductor layer 27 is a p-type semiconductor layer. In the light emitting cells C1, C2, C3, the first conductive type semiconductor layers 27 are separated from each other, so that each of the light emitting cells C1, C2, C3 constitutes each unit U1, U2, U3. A transparent electrode layer (not shown) may be placed on the second conductive type semiconductor layer 27.
The cathode 37 is placed on the first conductive type semiconductor layer 27 of each of the light emitting cells C1, C2, C3, and the anode 39 is also placed on the second conductive type semiconductor layer 27 of each of the light emitting cells C1, C2, C3. The cathodes 37 and the anodes 39 may be distributed in alternate arrangement over a wide area for current spreading.
Meanwhile, in adjacent light emitting cells, the cathode 37 is serially connected to the anode 39 via the interconnecting section 41. Accordingly, as shown in FIG. 8, it is possible to provide a light emitting diode having three serially connected light emitting cells and powered by, for example, a 12V power source.
In this light emitting diode, two first electrode pads 35 and two second electrode pads 33 are provided for current spreading, and the anode 39 and cathode 37 extend on each of the light emitting cells C1, C2, C3.
However, since the anode 39 and the cathode 37 are formed to relatively large sizes in order to obtain uniform current spreading on a large scale chip having, for example, a size of 1 mm×1 mm or more, optical loss inevitably occurs due to the anode and cathode 37, 39. Furthermore, since a relatively long anode 39 is placed on a single light emitting cell, current crowding is likely to occur near the second electrode pads 33. Moreover, since each light emitting cell has a relatively large light emitting area, light extraction efficiency is further deteriorated due to low current density.
On the other hand, for a light emitting diode in which a relatively large number of light emitting cells, for example, 15 light emitting cells, are serially connected in each of chips having the same size, the light emitting cells have small sizes so that current density increases in the light emitting cells, thereby deteriorating reliability.